1. Field of the Invention
The present invention relates to a semiconductor device and a display device. Specifically, the present invention relates to a semiconductor device and a display device which are lightweight; thin, and can realize a high performance at a low cost.
2. Description of the Related Art
Recently, research and development have eagerly been made on a display device for which a thin film transistor (TFT) is fabricated by using a polycrystalline silicon thin film as an active layer and provides as a switching element in a pixel portion, and an active matrix display device for which a circuit is provided so as to drive a pixel in the periphery of a pixel portion.
Furthermore, by using such high performance TFTs, research and development for providing a functional circuit represented by a large scale integrated circuit (LSI) over an insulating substrate have been started. The functional circuit conventionally could only be provided over a single crystalline semiconductor such as single crystalline silicon. It is to be noted that examples of such functional circuit include a central processing unit (CPU), a memory, an image processing circuit, a digital signal processor and the like. It is expected that a functional circuit which is thin, lightweight, and has low power consumption can be provided at a low cost by using TFTs. Therefore, a semiconductor device having a functional circuit formed by using TFTs and a display device in which a functional circuit formed by using TFTs is mounted on the same substrate are quite promising as future merchandises.
Generally, a CPU is expected to have a high processing performance. In the conventional development of LSI, processing performance of CPU has been improved by improving the operation frequency using multilayer wiring technology, for example. In the multilayer wiring technology, independent layers are employed in wirings in a basic cell, wirings in each block of a functional circuit, wirings between blocks, power supply wirings, ground wirings and the like. By forming such a multiplayer wiring, a chip area can be smaller and an operation speed can be higher. Furthermore, a memory is generally expected to have a high storage capacity and high read-out speed. In this case also, the multilayer wiring technology is very effective.